Cryogenic system

ABSTRACT

A cryogenic system includes a space formed between a cooling stage of a cryocooler unit and an element to be cooled, and a thermal joint placed in the space, wherein the thermal joint is composed of a substance that has a melting point higher than the cooling temperature of the element to be cooled and that is in a liquid or gaseous state at room temperature and atmospheric pressure. The cryogenic system can achieve reproducible and excellent thermal contact at a cooling stage without applying a large mechanical stress to a cryocooler unit structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cryogenic system for cooling asuperconducting magnet or the like.

2. Description of the Related Art

It is well known that a superconducting magnet apparatus must be kept atcryogenic temperatures to maintain the superconductivity. Examples ofcooling means include a method of immersing the superconducting magnetin a cryogen, such as liquid helium, and a method for cooling thesuperconducting magnet directly with a cryocooler without using acryogen.

FIG. 13 shows a structure of a conventional cryogenic system. This ismagnetic resonance imaging (MRI), which is known as a medical apparatus.FIG. 13 is a longitudinal section of the cryogenic system housing asuperconducting solenoid magnet having a horizontal central axis.

This cryogenic system includes a cryocooler unit 51, a vacuum chamber52, a superconducting magnet 53, a liquid helium bath 55, and aradiation-shield 56. The superconducting magnet 53 is placed in thevacuum chamber 52 and generates a large magnetic field. The liquidhelium bath 55 houses the superconducting magnet 53 and contains liquidhelium 54 for cooling the superconducting magnet 53. Theradiation-shield 56 is provided between the vacuum chamber 52 and theliquid helium bath 55 to shield radiation from the vacuum chamber 52 tothe liquid helium bath 55.

The cryocooler unit 51 has two stages; a first cooling stage 57 isthermally coupled to the radiation-shield 56 and a second cooling stage58 is thermally coupled to a recondenser 59, each stage being cooled ateach predetermined temperature. A sleeve 60 housing the cryocooler unit51 and the liquid helium bath 55 communicate with each other. Spacesover the liquid helium bath 55 and within the cryocooler sleeve 60 arefilled with helium having a saturated vapor pressure at an operatingtemperature of the superconducting magnet 53.

In such a helium recondensing cryogenic system, the cryocooler unit 51that has a cooling capacity at as low as 4.2K at the second coolingstage 58 is adopted.

Thus, the recondenser 59 has a surface temperature lower than thetemperature of liquid helium. When a gaseous phase of the liquid helium54 comes into contact with the recondenser 59, it can be recondensedinto liquid helium. Thus, in the system shown in FIG. 13, a user doesnot need to refill liquid helium 54 as long as the cryocooler unit 51operates, and can use the cryogenic system without monitoring acryogenic coolant.

However, the operation of the cryocooler unit 51 shown in FIG. 13 isinterrupted by periodical replacement of internal components ormaintenance of a compressor unit, which supplies compressed gas to thecryocooler unit 51. During such a maintenance, an increased amount ofheat is transferred to the liquid helium bath 55, promoting theevaporation of liquid helium 54. Furthermore, evaporated helium is notrecondensed by the cryocooler unit 51 during the maintenance and isentirely released from the system.

Thus, liquid helium 54 gradually decreases because of repeatedmaintenance and must be refilled. This increases the operation cost ofthe system. In the system shown in FIG. 13, therefore, the maintenancetime should be as short as possible to reduce the flow of heat into theliquid helium bath 55.

It should be noted that although the shutdown of the compressor does nottake many hours, the maintenance of the cryocooler unit 51 takesconsiderable time. That is, procedures of removing the cryocooler unit51 from the cryocooler sleeve 60; putting a new cryocooler unit 51 intothe cryocooler sleeve 60; starting the new cryocooler unit 51; andwaiting until a steady state is reached cannot be omitted.

Thus, in addition to the prompt replacement of the cryocooler unit 51,the reduction of the time to reach the steady state is required to beshorten. Thus, the maintenance cannot be shortened to a few hours.

The second problem in the maintenance of the cryogenic system relates tothermal resistance between the cryocooler unit 51 and an element to becooled.

In the structure shown in FIG. 13, to get the best performance from thecryocooler unit 51, a thermal contact between the first cooling stage 57and the radiation-shield 56, and a thermal contact between the secondcooling stage 58 and the recondenser 59 must be reproducibly maintainedin good condition. For example, a poor thermal contact between the firstcooling stage 57 and the radiation-shield 56 results in a largedifference in temperature at the interface (thermal contact resistance)when unit heat passes through the contact. This increases thetemperature of the radiation-shield 56 and therefore increases the flowof heat into the liquid helium bath 55. At worst, the flow of heat intothe liquid helium bath 55 exceeds the capacity of the recondenser 59;that is, not all the helium evaporated from the liquid helium 54 can berecondensed even when the cryocooler unit 51 is in operation. Thus,cryogen (liquid helium) must be added periodically. This considerablyimpairs the operationability of the cryogenic system.

A poor thermal joint between the second cooling stage 58 and therecondenser 59 is more serious. In the cryocooler unit 51 shown in FIG.13, since the second cooling stage 58 has a much smaller coolingcapacity than the first cooling stage 57, even a small difference intemperature at the second cooling stage 58 largely affects therecondensation. Even when both thermal joints have the same temperaturedifference of 1K, for example, this is obvious when the ratio of thetemperature difference to the operating temperatures of the firstcooling stage 5.7 and the second cooling stage 58 are considered.

Thus, when the temperature difference between the second cooling stage58 and the recondenser 59 is large, the temperature of the recondenser59 does not decrease sufficiently. At worst, cooling fins become higherin temperature than liquid helium 54 and cannot recondense helium gas.This also leads to the refilling of cryogen.

Increased thermal resistance between the cryocooler unit and an elementto be cooled partly results from contamination in the cryocooler sleeve60.

When the cryocooler unit 51 is removed from the cryocooler sleeve 60,the temperature inside the cryocooler sleeve 60 is lower than theambient temperature. In general, the temperature is 30 to 60K at theradiation-shield 56, and about 3 to 5K at the bottom of the cryocoolersleeve 60. Thus, when perfect measures are not taken to prevent theoutside air from entering the cryocooler sleeve 60, the air in theamount corresponding to the cryocooler unit 51 enters from the outsideof the vacuum chamber 52. This causes deposition of water vapor and airwithin the cryocooler sleeve 60. As a result, this decreases the contactarea at the interface between the first cooling stage 57 and theradiation-shield 56, increases the thermal resistance, and may cause thesame problem as described above.

In the past, many attempts to overcome these two big problems, that is,shortening of the maintenance and reduction of the thermal resistancebetween the cryocooler unit and the element to be cooled were made.

For example, one described in U.S. Pat. No. 5,918,470 is already known.

This prior art utilizes a hermetic liquid helium container, whicheliminates the addition of cryogen. Furthermore, a cooling stage of acryocooler unit and a recondenser of the liquid helium container (bath)are spaced at a predetermined interval. A thermal joint of an indiumgasket is placed in the gap to reduce thermal resistance between thecryocooler unit and an element to be cooled.

However, in the technique described in U.S. Pat. No. 5,918,470, to makeeffective use of the gasket to decrease the thermal resistance, veryhigh pressure must be applied to pinch the gasket. This may damage thecryocooler unit even with a soft metal gasket like indium.

Since the thermal contact resistance between two materials at cryogenictemperatures decreases as the contact pressure increases, the twomaterials should be pressed against each other at the highest possiblepressure to improve heat transfer at the contact surface of the thermaljoint. However, in general, the portion of the cryocooler unit betweenthe first cooling stage and the second cooling stage is often made ofvery thin material to reduce the heat flow. Thus, the cryocooler unitmay be damaged by a large mechanical stress.

On the contrary, an insufficient mechanical stress results in a largethermal resistance. Thus, it is very difficult to adjust the mountingposition of the cryocooler unit so that the mechanical stress would notbe too large and not too small.

Furthermore, when the cryocooler unit is removed for maintenance, in thecryogenic system with the indium gasket, heat may flow into the liquidhelium container from the gasket. Thus, it may take many hours to resumethe operation.

In addition, replacement of the gasket after removing the cryocoolerunit takes additional time for maintenance.

In an attempt to overcome the problem described above, U.S. Pat. No.6,164,077 proposed a heat transfer mechanism in which a liquid at acryogenic temperature is introduced into a thermal joint between asecond cooling stage and an element to be cooled, evaporates on theelement to be cooled, and recondenses at the second cooling stage.

The prior art described in U.S. Pat. No. 6,164,077 has many advantages.Since the thermal joint is achieved by the evaporation andrecondensation of a liquid coolant, a cryocooler unit can be easilyremoved and mounted. Moreover, since no stress is applied to the coolingstage, the precision of mounting position of the cryocooler unit is notan important issue.

However, for a superconducting magnet made of a superconducting metalwire as in this prior art, the element to be cooled must be maintainedat the liquid helium temperature or lower. Thus, a possible thermaljoint medium (cryogen) is only liquid helium. Furthermore, twomechanisms, that is, evaporation and recondensation of the cryogen arerequired in series between the element and the cryocooler unit. Thisresults in a larger difference in temperature and therefore lowerefficiency than the thermal transfer via a solid.

Furthermore, both techniques described in U.S. Pat. No. 5,918,470 andU.S. Pat. No. 6,164,077 cannot solve the problem of contamination in thecryocooler sleeve.

SUMMARY OF THE INVENTION

In consideration of these situations, an object of the present inventionis to provide a cryogenic system that reproducibly achieves excellentthermal contact at a cooling stage without applying a large stress to acryocooler unit structure.

Another object of the present invention is to provide a cryogenic systemin which a cryocooler unit is easily removed and mounted duringmaintenance.

Still another object of the present invention is to provide a cryogenicsystem that prevents a sleeve from being contaminated with the outsideair even when a cryocooler unit is removed, and that prevents heat fromflowing into an element to be cooled.

To achieve the objects described above, the present invention took thefollowing measures. A cryogenic system according to the presentinvention

In the structure described above, the substance of the thermal joint isin a solid state when the cryocooler unit is in operation. The substancemay be in a liquid or gaseous state when the cryocooler unit is at rest.

While thermal conductivity of any substance usually decreases remarkablywhen the substance changes in phase from solid to gas, a substance thathas the melting point of a cryogenic temperature and is in a gaseousstate at room temperature has a very high thermal conductivity, which issimilar to that of an alloy, at a solid state.

Thus, the thermal switch of the thermal joint according to the presentinvention is on when the cryocooler unit is in operation, and an elementto be cooled is cooled efficiently.

In a cryogenic system according to the present invention, when thecryocooler unit is removed from or mounted on the element to be cooled,the thermal joint can be in a liquid or gaseous phase. Thus, thecryocooler unit is easily removed or mounted. The cryocooler unit ismounted through the space and therefore is easily positioned.Furthermore, when the cryocooler unit is mounted on the thermal joint ina liquid or gaseous state, no stress is applied on the cryocooler unitand therefore possible damage of the cryocooler unit can be prevented.

The substance of the thermal joint contains at least one selected fromthe group consisting of nitrogen, neon, para-hydrogen, and water.

The cryogenic system may comprise a sleeve housing the thermal joint.

Preferably, heating means for heating the thermal joint is provided.

This heating means can force phase conversion of the thermal joint fromsolid to liquid or gas and thus shorten the time of removing thecryocooler unit.

Preferably, cryogenic system comprises a buffer tank for storing thesubstance, and the buffer tank is coupled to the thermal joint via aconnecting pipe.

Preferably, the substance stored in the buffer tank is hydrogen, and acatalyst vessel containing a catalyst for ortho-para hydrogen conversionis intervened by the connecting pipe.

The element to be cooled may have a liquid helium container including asuperconducting magnet and liquid helium. The sleeve may be provided onthe liquid helium container. An enclosed recondenser may be verticallyprovided in the sleeve. The thermal joint may be provided on therecondenser.

The circumference of the element to be cooled may be covered with aradiation-shield, the circumference of which may be covered with avacuum chamber. The sleeve may be provided between the vacuum chamberand the element to be cooled. The cooling stage may be provided in thesleeve in a manner such that the cooling stage can be inserted andremoved as desired.

The cooling stage includes a first cooling stage on the side of highertemperatures and a second cooling stage on the side of lowertemperatures. Preferably, the first cooling stage cools theradiation-shield and the second cooling stage cools the element to becooled, and the first cooling stage and/or the second cooling stage isprovided with the thermal joint.

The element to be cooled may be a superconducting magnet.

Preferably, a surface of the element to be cooled, in contact with thethermal joint, is made of a first material having a high thermalconductivity, such as copper, and the sleeve is made of a secondmaterial having a lower thermal conductivity than the first material,such as stainless steel, and the sleeve is provided with a deformationabsorber for absorbing vertical deformation.

The present invention provides a cryocooler unit that reproduciblyachieves excellent thermal contact without applying a large stress to acryocooler unit structure.

Furthermore, the cryocooler unit can be easily removed and mountedduring maintenance.

In addition, even when the cryocooler unit is removed, heat can hardlyflow into an element to be cooled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a cryogenic system according toan embodiment of the present invention.

FIG. 2 is an enlarged view of a principal part according to anotherembodiment of the present invention.

FIG. 3 is an explanatory view of the operation according to theembodiment of the present invention shown in FIG. 2.

FIG. 4 is an explanatory view of the operation according to theembodiment of the present invention shown in FIG. 2.

FIG. 5 is an explanatory view of the operation according to theembodiment of the present invention shown in FIG. 2.

FIG. 6 is a schematic sectional view of a cryogenic system according toanother embodiment of the present invention.

FIG. 7 is a schematic sectional view of a cryogenic system according toanother embodiment of the present invention.

FIG. 8 is a schematic sectional view of a cryogenic system according toanother embodiment of the present invention.

FIG. 9 is an enlarged view of a principal part according to anotherembodiment of the present invention.

FIG. 10 is an enlarged view of a principal part according to anotherembodiment of the present invention.

FIG. 11 is a schematic sectional view of a cryogenic system according toanother embodiment of the present invention.

FIG. 12 is an enlarged view of a principal part according to anotherembodiment of the present invention.

FIG. 13 is a schematic sectional view of a conventional cryogenicsystem.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments according to the present invention will now be describedwith reference to the drawings.

FIG. 1 is a schematic sectional view of one example of a cryogenicsystem, MRI. This cryogenic system includes a cryocooler unit 1 and anelement to be cooled 2. The cryocooler unit 1 includes a cooling stage3. A space is formed between the cooling stage 3 and the element to becooled 2, and a thermal joint 4 is placed in the space. The thermaljoint 4 is composed of a substance that has the melting point higherthan the cooling temperature of the element to be cooled 2 and that isin a liquid or gaseous state at room temperature and atmosphericpressure. The thermal switch of the thermal joint 4 is on in a solidstate.

The exemplary cooling stage 3 includes a first cooling stage 5 and asecond cooling stage 6 in series. The second cooling stage 6 has atemperature of about 4K, which is lower than that of the first coolingstage 5. The second cooling stage 6 has a cooling capacity lower thanthat of the first cooling stage 5.

The element to be cooled 2 includes a liquid helium container 9including a superconducting magnet 7 and liquid helium 8. Thecircumference of the liquid helium container 9 is covered with aradiation-shield 10. The circumference of the radiation-shield 10 iscovered with a vacuum chamber 11. A sleeve 12 is provided between thevacuum chamber 11 and the helium container 9.

The sleeve 12 is cylindrical and is provided through theradiation-shield 10. The cryocooler unit 1 is provided so as to placethe cooling stage 3 of the cryocooler unit 1 in the sleeve 12 in amanner such that the cryocooler unit 1 can be inserted and removed asdesired. A space is formed between an end face of the second coolingstage 6 of the cooling stage 3 and the liquid helium container 9. Thethermal joint 4 is formed in the space.

The vacuum chamber 11 has a circular internal space formed by an innerwall 13 and an outer wall 14, which are concentrically disposed at apredetermined interval, and by side walls 15, 15, which connect theinner wall and the outer wall. The vacuum chamber 11 is disposed so asto have a horizontal central axis. A center space formed by the circularinner wall 13 has openings at both right and left ends.

The radiation-shield 10 and the liquid helium container 9 housed in thecircular space of the vacuum chamber 11 are also cylindrical and areclosed at both ends, as with the vacuum chamber 11. A superconductingmagnet 7 housed in the liquid helium container 9 is also cylindrical.The superconducting magnet 7, the liquid helium container 9, theradiation-shield 10, and the vacuum chamber 11 are disposedconcentrically.

The circular space within the vacuum chamber 11 is maintained at apredetermined vacuum pressure, and the space within the radiation-shield10 is also maintained at the same pressure.

A space is formed between the surface of liquid helium 8 in the liquidhelium container 9 and the inner surface of the liquid helium container9. The space is filled with helium gas 16 at a saturated vapor pressure.

The lower end of the sleeve 12 is hermetically connected to the top ofthe liquid helium container 9. The sleeve 12 and the liquid heliumcontainer 9 are integrated into one piece by welding or otherprocessing. An enclosed recondenser 17 is vertically provided in thesleeve 12.

The recondenser 17 is provided with a shield plate 18, which is tightlycoupled to the inner surface of the sleeve 12, and a fin 19 suspendedfrom the undersurface of the shield plate 18. The recondenser 17 may befixed to the liquid helium container 9, instead of being fixed to theinner surface of the sleeve 12.

The recondenser 17 is made of a material having a high thermalconductivity, such as copper. The sleeve 12 is made of a material havinga low thermal conductivity, such as stainless steel. The upper end ofthe sleeve 12 has an opening in the surface of the vacuum chamber 11.The cooling stage 3 of the cryocooler unit 1 is inserted from thisopening. The opening at the upper end of the sleeve 12 can be sealed byfitting the cryocooler unit 1.

The first cooling stage 5 in the sleeve 12 is thermally coupled to theradiation-shield 10 and chills the radiation-shield 10. A predeterminedspace is formed between the undersurface of the second cooling stage 6and the top surface of the shield plate 18 of the recondenser 17. Thisspace is filled with the substance of the thermal joint 4.

The thermal joint 4 is composed of the substance, which has the meltingpoint higher than the cooling temperature of the element to be cooled 2and that is in a liquid or gaseous state at room temperature andatmospheric pressure. The amount of the substance in a solid state issufficient to thermally connect the second cooling stage 6 and therecondenser 17. Specifically, the substance of the thermal joint 4preferably contains at least one selected from the group consisting ofnitrogen, neon, para-hydrogen, and water. More preferably, the substancemainly contains these components solely or in combination. The substancecontaining only one of these compounds in a high purity (for example,99.99% or more) has an excellent thermal conductivity and thus is moresuitable.

According to the embodiment of the present invention, the substance ofthe thermal joint 4 is nitrogen. In the operational status shown in FIG.1, the thermal joint 4 is in a solid state, and nitrogen gas is presentat a saturated vapor pressure within the sleeve 12.

In general, the thermal conductivity of solid nitrogen reaches its peakof about 20 W/m/K at 4.2K, at which the superconducting magnet 7 isoperated. This value is comparable to that of an alloy, such as solder.Thus, the temperature difference between the second cooling stage 6 andthe recondenser 17 can be greatly reduced. For example, when the spacebetween the second cooling stage 6 and the recondenser 17 is 1×10⁻⁴ m,the cooling capacity of the second cooling stage 6 is 1 W, and the heattransfer area is 0.005 m², then the temperature difference will be only0.001K. Thus, thermal contact that is comparable to that in the casewhere the second cooling stage 6 is connected to the recondenser 17 viaa metal can be obtained.

The cryocooler unit 1 is shut down when it is removed for maintenance orfixing. This increases the temperature of the thermal joint 4 and causessolid nitrogen to melt, thus separating the second cooling stage 6 andthe recondenser 17.

To mount the cryocooler unit 1, liquid nitrogen is charged into thesleeve 12 to form a liquid layer having a predetermined thickness on therecondenser 17. Then, the cryocooler unit 1 is mounted inside the sleeve12, and the lower end of the second cooling stage 6 is immersed inliquid nitrogen. After the cryocooler unit 1 is started, liquid nitrogenis cooled by the second cooling stage 6 into solid. Through the thermaljoint 4 of this solid nitrogen, the second cooling stage 6 is thermallycoupled to the recondenser 17.

FIG. 2 is another embodiment of the present invention. Like componentsare denoted by like numerals in FIG. 1. Unlike the embodiment in FIG. 1,heating means 20 for heating a substance of a thermal joint. 4 in asleeve 12 is provided.

The heating means 20 includes a gas feed pipe 21 connected to the sleeve12. One end of the gas feed pipe 21 has an opening in the sleeve 12 inthe vicinity of the thermal joint 4, and the other end is disposedoutside a vacuum chamber 11 and is provided with a gas feed valve 22.The gas feed valve 22 is connected to a nitrogen gas feeder (not shown).Nitrogen gas supplied from the nitrogen gas feeder heats the thermaljoint 4 and melts the solid nitrogen.

The structure of the sleeve 12 according to this embodiment is differentfrom that shown in FIG. 1.

That is, the sleeve 12 consists of an upper sleeve 23 surrounding afirst cooling stage 5 and a lower sleeve 24 surrounding a second coolingstage 6.

The upper sleeve 23 is composed of a circular poor thermal conductor.The upper sleeve 23 has at its upper end an upper flange 25 radially andoutwardly protruding. Between the upper end and the lower end, the uppersleeve 23 is provided with a deformation absorber 26 for absorbingvertical deformation. This deformation absorber 26 has a pleated shape.The upper opening of the upper sleeve 23 is hermetically fitted to a toplid 27 with an 0-ring 28 therebetween. The top lid 27 is fixed on theupper flange 25 in a manner such that it can be attached and removed asdesired.

The cryocooler unit 1 is fixed on the top lid 27. Bolts 29 for adjustingthe height of the top lid 27 relative to the vacuum chamber 11 areprovided in the upper flange 25. The top lid 27 is provided with a checkvalve 30 for maintaining a constant pressure when the internal pressureof the sleeve 12 exceeds the atmospheric pressure by a predeterminedvalue.

A lower flange 31 is connected to the lower end of upper sleeve 23. Thislower flange 31 is composed of a good thermal conductor and is incontact with the first cooling stage 5. The lower flange 31 is thermallycoupled to a radiation-shield 10 via a copper braid 32.

The lower sleeve 24 is composed of a poor thermal conductor and isconnected at its upper end to the lower flange 31. A deformationabsorber 26 in a pleated shape is disposed under the lower sleeve 24 andis connected at its lower end to a liquid helium container 9.

The lower sleeve 24 is internally sectioned into hermetic upper andlower halves by a shield plate 18 at the level higher than thedeformation absorber 26. This shield plate 18 is composed of a goodthermal conductor. A fin 19 is suspended from the undersurface of theshield plate 18. The shield plate 18 and the fin 19 constitute arecondenser 17.

A space is formed between the upper surface of the recondenser 17 andthe lower surface of the second cooling stage 6, and is filled with thesubstance of the thermal joint 4. The gas feed pipe 21 is connected tothe side of the sleeve 24 near the bottom of the thermal joint 4. Thegas feed pipe 21 passes through the vacuum chamber 11 in a compartmentbeing at room temperature and is connected to a gas feed valve 22outside the cryogenic system. Thus, heating gas can be introduced intothe vicinity of the sleeve 12 from the outside of the system.

FIGS. 3 to 5 are explanatory views of the operation of the system shownin FIG. 2.

When precooling starts, the sleeve 12 is in a state shown in FIG. 3. Thesleeve 12 is opened at its upper end and communicates with theatmosphere. The element to be cooled is precooled with cryogen.According to the present invention, evaporated gas of cryogen generatedby the precooling of the element to be cooled is not discharged from thesleeve 12 into the atmosphere. Thus, even when the liquid heliumcontainer 9 is filled with liquid helium 8, the recondenser 17 in thesleeve 12 has a considerably high temperature (for example, thetemperature of liquid nitrogen or higher).

Then, as shown in FIG. 4, liquid nitrogen 33 is charged into the sleeve12.

Then, the cryocooler unit 1 is mounted inside the sleeve 12. The secondcooling stage 6 of the cryocooler unit 1 is immersed in liquid nitrogen33 and is precooled. Liquid nitrogen 33 absorbs heat from the secondcooling stage 6 to evaporate. The evaporated nitrogen is discharged fromthe opening at the upper end of the sleeve 12 into the atmosphere. Whileliquid nitrogen 33 evaporates rapidly, the top lid 27 of the sleeveshould not completely be attached to the upper flange 25 so that theinterior of the sleeve can communicate with the atmosphere to a certainextent.

When the second cooling stage 6 is completely precooled and theevaporation rate of the liquid nitrogen 33 almost reaches a constantvalue, nitrogen gas is fed from the gas feed valve 22 via the gas feedpipe 21 to the bottom of the sleeve 12 to evaporate liquid nitrogen 33so that liquid nitrogen 33 remains only between the second cooling stage6 and the recondenser 17, as shown in FIG. 5.

Then, the top lid 27 is completely attached to the upper flange 25 toseal the sleeve 12. Then, the cryocooler unit 1 is started. Once thesecond cooling stage 6 of the cryocooler unit 1 has already beensufficiently precooled, the cooling stage 3 can immediately reach asteady-state operating temperature.

When the second cooling stage 6 decreases in temperature, liquidnitrogen is cooled and solidifies in a short time. Most parts within thesleeve 12 contain only a small amount of gas at a saturated vaporpressure.

In this way, the second cooling stage 6 cools the recondenser 17 via thethermal joint 4.

The following is a procedure for replacing the cryocooler unit 1.

First, the cryocooler unit 1 is brought to a halt. Nitrogen gas is thenfed from the gas feed valve 22 via the gas feed pipe 21 to increase thetemperature of the lower sleeve 24. Nitrogen gas introduced into thesleeve 12 melts and evaporates solid nitrogen in the thermal joint 4,thereby separating the second cooling stage 6 from the shield plate 18,and finally escapes from the check valve 30 of the top lid 27 into theatmosphere. Since the recondenser 17 and the liquid helium container 9are separated by the lower sleeve 24 being a poor thermal conductor,heat flowing into the liquid helium container 9 is reduced, so that theentire liquid helium does not reach 4.2K during maintenance.

When solid nitrogen around the second cooling stage 6 evaporates and iscompletely removed, the cryocooler unit 1 and the top lid 27 are liftedup to remove the cryocooler unit 1. During this removal, nitrogen gas iscontinuously supplied from the gas feed pipe 21 to reduce the aircontamination in the sleeve 12 to a minimum. When cryocooler unit 1 iscompletely removed, the temperature of the lower sleeve 24 at levelshigher than the shield plate 18 has already reached about 80K, forexample.

Then, in a similar way to the precooling, liquid nitrogen is fed intothe sleeve 12. In this case, since the internal temperature of thesleeve 12 is sufficiently low, liquid nitrogen does not evaporate inlarge quantity.

Then, a new cryocooler unit 1 is placed in the sleeve. From then on,procedures similar to those of precooling the cryostat is followed, andfinally the cryocooler unit 1 is started. As is the case with theprecooling, the interior of the sleeve 12 is already precooledsufficiently and therefore the cryocooler unit 1 can immediately reach asteady-state operating temperature.

According to the embodiment described above, the gas feed pipe 21 avoidspossible air contamination in the sleeve 12 during the replacement ofthe cryogenic system and thus prevents the degradation of the thermalcontact, which should be kept clean.

Furthermore, since solid nitrogen used as the substance of the thermaljoint 4 has a heat transfer coefficient similar to that ofphosphorous-deoxidized copper at temperatures around 4.2K, the thermaljoint 4 can be smaller than a copper brade, and thus the temperaturedifference in the thermal joint 4 can be reduced. In addition, thesystem according to the embodiment of the present invention can increasethe temperature locally, and evaporate only solid nitrogen to make thesecond cooling stage 6 detachable, allowing for immediate replacement ofthe cryocooler unit 1.

FIG. 6 shows another embodiment of the present invention, in which acryogenic system is directly cooled with a cryocooler unit and anelement to be cooled 2 is not immersed in cryogen.

The element to be cooled 2, such as a superconductive magnet, is housedin a vacuum chamber 11 and is cooled with a second cooling stage 6 ofthe cryocooler unit 1. Furthermore, to reduce the thermal radiation fromthe vacuum chamber 11 to the element to be cooled 2, a radiation-shield10 is provided between the vacuum chamber 11 and the element to becooled 2, and is cooled with a first cooling stage 5. The cryocoolerunit 1 is housed in the sleeve 12, which is provided at the lower endwith a bottom lid 34. The bottom lid 34 is thermally coupled to thesecond cooling stage 6 of the cryocooler unit 1 via a thermal joint 4.

This embodiment has the same structure with that in FIG. 2, except thatthe element to be cooled 2 is not indirectly cooled by the action of arecondenser. Thus, the same procedures described for FIG. 2 can beapplied to the replacement of the cryogenic system.

Thus, according to the present embodiment, in the replacing procedure ofthe cryogenic system, the element to be cooled is only heated to about80K. This does not require a large amount of heat, unlike theconventional method in which the temperature is increased to roomtemperature. Thus, economical and rapid maintenance can be performed.

FIG. 7 shows another embodiment of the present invention.

This embodiment differs from the other embodiments described above inthat a sleeve 12 is connected to a buffer tank 35, which contains asubstance of a thermal joint 4, and that heating means 20 is a heaterprovided in the vicinity of a thermal joint 4.

The buffer tank 35 is fixed on the inner surface of a radiation-shield10 and communicates with the interior of a sleeve 12 via a connectingpipe 36, so that the substance can flow between the sleeve 12 and thebuffer tank 35.

The amount of the substance of the thermal joint 4 is adjusted so thatwhen it is solidified a solid phase is formed only in the thermal joint4 and the buffer tank 35 is filled with gas.

The heating means 20 can heat the thermal joint 4 to a desiredtemperature.

According to this embodiment, when the cryocooler unit 1 is removed, theheating means 20 is operated to maintain the substance of the thermaljoint 4 at a temperature such that the substance is converted into gas.The substance expands through this operation and the excess substanceflows into the buffer tank 35. When the internal volume of the buffertank 35 is about 100 times as large as that of the thermal joint 4, thisdoes not increase the internal pressure significantly. Deductively, theinternal pressure after the gasification may be determined based onwithstanding pressures of the buffer tank 35 and the sleeve 12.

After the new cryocooler unit 1 is mounted and started, it takesadditional time for the first cooling stage 5 and the second coolingstage 6 of the cryocooler unit 1 to reach each steady-state operatingtemperature. However, in the second cooling stage 6, since the thermalswitch of the thermal joint 4 is off until the steady-state operatingtemperature is reached, heat transfer to a recondenser 17 and a liquidhelium container 9 can be reduced to minimum.

When the temperature of the second cooling stage of the cryocooler unit1 reaches a temperature between the melting point and the boiling pointof the substance of the thermal joint 4, a preset temperature of theheating means 20 is changed to this temperature. When the cryocoolerunit 1 is in operation, the temperature of the thermal joint 4immediately decreases, and the substance stored mainly in the buffertank 35 is liquefied at the thermal joint 4. During the liquefaction,generated heat of condensation is absorbed by the cryocooler unit 1.After the thermal joint 4 is filled with the substance, the heatingmeans 20 is brought to a halt. Then, the substance of the thermal joint4 immediately solidifies and returns to the state before themaintenance.

FIG. 8 shows another embodiment of the present invention, in which acryogenic system is directly cooled with a cryocooler unit and has thestructure shown in FIG. 7. An element to be cooled 2 is not immersed incryogen.

FIG. 9 shows a cryogenic system according to another embodiment of thepresent invention. A substance stored in the buffer tank 35 is hydrogen.A catalyst vessel 37 containing a catalyst for ortho-para hydrogenconversion is connected to the buffer tank 35 via the connecting pipe36. The catalyst vessel 37 is thermally coupled to a second coolingstage via a thermal conductor 37 a. The buffer tank 35 is thermallycoupled to a first cooling stage 5 via a thermal conductor 35 a. Thebuffer tank 35 is connected to a supply pipe 35 b, which extends acrossthe vacuum chamber 11. Because of the catalyst in the catalyst vessel37, para-hydrogen is converted into ortho-para hydrogen which is used asa substance of a thermal joint.

The thermal conductivity of para-hydrogen is more than 1 W/cm/K at about4K, about 0.008 W/cm/K at about 20K near the boiling point, and 0.0004W/cm/K at about 40K, over the boiling point. The conductivity ratioreaches 2500:1. Thus, heat flow vertically passing through the thermaljoint 4 decreases to 1/2500 when the temperatures at upper and lowerends are identical, and thereby thermal switch of off state can beachieved.

FIG. 10 shows another embodiment of the present invention, in which afirst cooling stage 5 is also provided with a thermal joint 38. That is,the thermal joint 38 is formed so as to fill a space provided betweenthe first cooling stage 5 and a radiation-shield 10.

More specifically, a toroidal sleeve 39 is provided between the firstcooling stage 5 and the radiation-shield 10, and is filled with asubstance of the thermal joint 38. The sleeve 39 is connected to abuffer tank 41 via connecting pipes 40. This buffer tank 41 is mountedon the inner surface of a vacuum chamber 11.

The substance of the thermal joint 38 at the first cooling stage 5 isdifferent from a substance of a thermal joint 4 at a second coolingstage 6, and has a higher melting point than the substance of thethermal joint 4. For example, when a steady-state operating temperatureof the radiation-shield 10 is 60K, the substance of the thermal joint 4at the second cooling stage 6 is para-hydrogen, and the substance of thethermal joint 38 at the first cooling stage 5 is water.

The buffer tanks 35 and 41 is connected to supply pipes 42 extendingacross the vacuum chamber 11.

FIG. 11 shows another embodiment of the present invention, in which acryogenic system is directly cooled with a cryocooler unit and anelement to be cooled 2 is not immersed in cryogen. The cryogenic systemhas two thermal joints 4; that is, each thermal joint 4 is provided oneach side of a bottom lid 43 of a sleeve 12.

FIG. 12 shows a cryogenic system using liquid helium, according toanother embodiment of the present invention.

In this embodiment, the bottom of a sleeve 12 has a funnel shape. Thesleeve 12 is internally sectioned into an upper half and a lowercondensing chamber 44 by a recondenser 17. The lower end of thecondensing chamber 44 and the top of a liquid helium container 9 areconnected through a condensate-supply pipe 45. The upper part of thecondensing chamber 44 and the top of the liquid helium container 9 areconnected through an evaporated gas-supply pipe 46.

A thermal joint 4 is formed between the top of the recondenser 17 andthe bottom of a second cooling stage 6.

When the temperature of the recondenser 17 is lower than the temperatureof gas in the condensing chamber 44, helium liquefies on the surface ofa fin 19 of the recondenser 17, is collected to the bottom, and returnsto the liquid helium container 9 through the condensate-supply pipe 45.Concurrently, gas in the liquid helium container 9 is automatically fedto the condensing chamber 44 through the evaporated gas-supply pipe 46.

In the system shown in FIG. 12, since the condensing chamber 44 isconnected to the liquid helium container 9 via the condensate-supplypipe 45 and the evaporated gas-supply pipe 46 each having a smallcross-section, heat transfer from the condensing chamber 44 to theliquid helium container 9 is reduced to a minimum.

The present invention is not limited to each embodiment described above.In addition, it is needless to say that various modifications may bemade without departing from the gist of the present invention. Forexample, the present invention may be applied to NMR.

The present invention can be utilized in medical apparatus industries,such as MRI, and precision analysis instrument industries, such as NMR.

1. A cryogenic system comprising: a cryocooler unit; an element to becooled; a cooling stage of said cryocooler unit, said cooling stagebeing provided so that a space formed between said cooling stage of saidcryocooler unit and said element to be cooled; and a thermal jointplaced in said space, wherein said thermal joint is composed of asubstance that has a melting point higher than the cooling temperatureof said element to be cooled and that is in a liquid or gaseous state atroom temperature and atmospheric pressure, and wherein said thermaljoint functions as a thermal switch which is on when the substance is ina solid states the substance of the thermal joint is in a solid statewhen the cryocooler unit is in operation, and in a liquid or gaseousstate when the cryocooler unit is at rest, and the substance contains atleast one selected from the group consisting of nitrogen, neon,para-hydrogen, and water.
 2. (canceled)
 3. The cryogenic systemaccording to claim 1, further comprising a sleeve housing said thermaljoint.
 4. The cryogenic system according to claim 1, further comprisingmeans for heating said thermal joint.
 5. The cryogenic system accordingto claim 1, further comprising a buffer tank for storing the substance,and a connecting pipe, wherein said buffer tank is connected to saidthermal joint via said connecting pipe.
 6. A cryogenic system comprisinga cryocooler unit; an element to be cooled; a cooling stage of saidcryocooler unit, said cooling stage being provided so that a spaceformed between said cooling stage of said cryocooler unit and saidelement to be cooled; a thermal joint placed in said space; a buffertank for storing the substance; a connecting pipe; and a catalyst vesselcontaining a catalyst, wherein said thermal joint is composed of asubstance that has a melting point higher than the cooling temperatureof said element to be cooled and that is in a liquid or gaseous state atroom temperature and atmospheric pressure, and wherein said thermaljoint functions as a thermal switch which is on when the substance is ina solid state, wherein said buffer tank is connected to said thermaljoint via said connecting pipe, and wherein the substance stored in saidbuffer tank is hydrogen, the catalyst is for ortho-para hydrogenconversion and said catalyst vessel is intervened by said connectingpipe.
 7. The cryogenic system according to claim 3, wherein said elementto be cooled includes a liquid helium container including asuperconducting magnet and liquid helium, said sleeve is provided onsaid helium container, an enclosed recondenser is vertically provided insaid sleeve, and said thermal joint is formed on said recondenser. 8.The cryogenic system according to claim 3, further comprising aradiation-shield covering said element to be cooled and a vacuum chambercovering the circumference of said radiation-shield, wherein said sleeveis provided between said vacuum chamber and said element to be cooled,and said cooling stage is provided in said sleeve in a manner such thatsaid cooling stage can be inserted and removed as desired.
 9. Thecryogenic system according to claim 1, wherein said cooling stageincludes a first cooling stage on the side of higher temperatures and asecond cooling stage on the side of lower temperatures, said firstcooling stage cooling said radiation-shield and said second coolingstage cooling said element to be cooled, wherein at least one of saidfirst cooling stage and said second cooling stage is provided with saidthermal joint.
 10. A cryogenic system, comprising: a cryocooler unit; anelement to be cooled; a cooling stage of said cryocooler unit, saidcooling stage being provided so that a space formed between said coolingstage of said cryocooler unit and said element to be cooled; a thermaljoint placed in said space; and a sleeve housing said thermal joint,wherein said thermal joint is composed of a substance that has a meltingpoint higher than the cooling temperature of said element to be cooledand that is in a liquid or gaseous state at room temperature andatmospheric pressure, and wherein said thermal joint functions as athermal switch which is on when the substance is in a solid state, andwherein a surface of said element to be cooled, in contact with saidthermal joint, is made of a first material, and said sleeve is made of asecond material having a lower thermal conductivity than the firstmaterial, and said sleeve is provided with a deformation absorber forabsorbing vertical deformation.
 11. The cryogenic system according toclaim 5, further comprising a catalyst vessel containing a catalyst,wherein the substance stored in said buffer tank is hydrogen, thecatalyst is for ortho-para hydrogen conversion and said catalyst vesselis intervened by said connecting pipe.
 12. The cryogenic systemaccording to claim 3, wherein a surface of said element to be cooled, incontact with said thermal joint, is made of a first material, and saidsleeve is made of a second material having a lower thermal conductivitythan the first material, and said sleeve is provided with a deformationabsorber for absorbing vertical deformation.